Such a method is e.g. known from DE 196 26 967 C2. It is also disclosed in this document that the at least one wall opening is formed in that the wall of the outer container is pierced or punctured by a drill, graver or a piercing needle, a pressure medium being blown or injected against the wall of the inner pouch after complete penetration of the wall of the outer container to press the pouch back so that it is not damaged by the drill or piercing needle. The pressure medium may be air, gas, water or a gel.
Although the different thermoplastic materials of the outer container and the inner pouch do not form a welded joint with one another, they adhere to each other when the container is produced in a coextrusion process according to the above-described method. Before such a container can be used, i.e. before being filled with a liquid or, for example, also gel-like container contents which is then gradually discharged, for example, by means of an airless pump or in the case of a squeeze container by the container being squeezed, the inner pouch must be removed or detached at least for its greatest part from the wall of the outer container and must then be placed on the wall again. This has so far been done by applying a negative pressure or vacuum to the inner pouch through the container opening, the inner pouch thereupon contracting suddenly. In this process the inner pouch which is clamped with its bottom seam in the bottom web of the outer container detaches from the outer container in a more or less uncontrolled manner, and it may e.g. happen that the inner pouch only detaches at one side. Subsequently a pressure medium, in general compressed air, is introduced through the container opening into the inner pouch to bring said pouch again into contact with the outer container so that the inner pouch has the intended filling volume. When the container has the shape of a bottle with a neck which passes through shoulder sections into the e.g. cylindrical body of the outer container, it is often unavoidable under the known procedure that air gets entrapped in the transition area of the shoulder to the main body of the outer container between the outer container and the inner pouch, which air can no longer escape. This also happens whenever the body of the outer container does not extend along a straight line, e.g. cylindrically, but has indentations and bulges.
In cases where the inner pouch has not been detached from the wall of the outer container in a substantially uniform manner, a relatively high negative pressure arises, despite pressure compensating openings in the wall of the outer container, in the inner pouch due to the discharge of filling material. The higher this negative pressure, the greater is the permeation through the wall of the inner pouch and the greater is the risk that the pouch will leak. If the negative pressure in the inner pouch is too high, this may entail inoperativeness of the pump, which is most of the time provided, so that it is not possible to discharge the whole contents of the container. It may also happen that the negative pressure is so high at the beginning that outer bottle and inner bottle will fold at the same time until the negative pressure reaches a value which enables the inner pouch to detach suddenly.